The present invention generally relates to a lithium ion battery cathode, and in particular to a method of optimizing granular structure and performance of a lithium ion battery cathode.
The development of lithium-ion batteries with high energy density and discharge/recharge rates is important to their utilization in transportation, energy storage, and portable electronics applications. In addition, development of lithium ion batteries with dense, three-dimensional architectures can enable micro-powered, micro-electromechanical systems and independently powered ‘smart dust’ particles to enable highly distributed computing.
Lithium cobalt oxide batteries, and in particular LiCoO2-graphite based batteries, have enjoyed commercial success since being introduced to the market. However, the limited availability of cobalt has hindered its economic viability for high-power applications such as hybrid-electric vehicles and portable electronic applications, each of which can require a large amount of charge storage. Due to its limited availability, cobalt can be expensive and thus less commercially viable for these industries. As a result, lithium iron phosphate, LiFePO4, has been analyzed more extensively as a potential replacement for the development of low-cost, environmentally inert lithium iron batteries. As described below, LiFePO4, has different material properties that can affect lithium ion diffusion. In particular, lithium ions can diffuse one-dimensionally and preferentially along one direction in the crystal. This direction is typically the direction in the crystal due to crystal structure and chemical interaction therein.
During a discharging process (e.g., where power is being delivered from the battery), an anode of the battery serves as a source of lithium ions to be inserted into a solid cathode by transporting across an ionically-conducting electrolyte, while electrons flow from the anode to cathode through an external circuit. Anodes are typically composed of a graphite material that is intrinsically, electrically conductive. Cathodes are commonly fabricated with lithium-containing compounds, often referred to as insertion materials. In contrast, lithium oxide compounds have recently become popular cathode materials. Cathode composition limits the amount of ionic charge that can be reversibly stored in the battery.
The one-dimensional nature of ion diffusion in LiFePO4 has been the subject of recent experimental and theoretical studies. In addition, during synthesis of LiFePO4 particles, the particles have shown a tendency to form facets perpendicular to the highly ion conducting direction, namely the [010] direction. Accordingly, attempts have been made to maximize surface area of particles along that direction through the adjustment of synthesis conditions. Despite possessing 10% higher gravimetric energy density than LiCoO2, LiFePO4 can possess 20% lower volumetric energy density than LiCoO2. Therefore, to exceed the effective volumetric energy density of current batteries, it is desirable to fabricate densely packed microstructures which are generally synthesized through a wet chemical process to achieve sufficiently high volumetric density of energy storage. Such requirements, however, may limit the economic viability of cathode architectures exhibiting low packing density. Although the highly porous structure may result in high gravimetric density and high rate performance, visual investigation of SEM images reveals that the solid density is actually less than 30%. The inhibition of electrolyte penetration and subsequent reduction of electrochemical performance has been found due to the agglomeration of particles.
Experimentally, it has been found that if the size of the LiFePO4 particles is reduced, the diffusion length scales through which ions diffuse is also reduced. Thus, higher diffusion rates are possible which enables LiFePO4 particles to be used for battery applications. In addition, different shapes of LiFePO4 particles can be synthesized including nanoplatelets and equiaxed particles. The smallest sized shape that has been synthesized is the [010] platelet. Low aspect ratio plate-shaped particles, however, have shown a tendency to exhibit columnar ordering. As a result of the tendency of LiFePO4 to form facets normal to [010], the fast lithium ion diffusion direction, such ordering can prevent or inhibit access of liquid electrolyte to the highly active surfaces. In other words, columnar ordering can reduce ion transport due to an increase in the diffusion length scales for ions in the material, and thereby a reduction in diffusion rate. A reduction in diffusion rate can negatively impact charge storage and thereby the commercial viability of LiFePO4 particles for use in the cathode material.
A need therefore exists for a method to reduce or eliminate the effects of columnar ordering and provide a means for effective ion diffusion for commercially-viable applications.